I think it depends on the saddle, the jeans, the rider, and the activity. Say, for example, a very soft French leather Butet, some tough & slightly baggy jeans, a somewhat off-balance rider, and a jumping course. I do believe the bunched up, baggy jeans could rub along the seams of the twist and cause damage. I've seen it many, many times. If you look on eBay
, many of the nice french saddles (highly used ones) will have splits along the seams of the twist (or just major wear); this can be attributed to riding in jeans, among other things.
Of course, metal grommets on jeans pockets are also a no-no!
I always ride in breeches. I feel like I can't move my legs freely in jeans. (Or maybe my jeans are too tight--haha!)
PS-I think riding in jeans in a Western saddle is fine. They are built differently. And as others have said, people have been doing it for ages!